(a) Field of the Invention
The present invention relates to a polypropylene-based resin composition which can be suitably used as resin material for production of polypropylene-based resin sheets, the polypropylene-based resin sheets being usable, as molding material, to production of various packaging containers or the like.
The present invention also relates to a method for preparation of polypropylene-based resin sheets by using the above-described polypropylene-based resin composition.
(b) Description of the Related Art
As polypropylene-based resins have excellent strength, rigidity, and heat resistance, they are used for packaging foods, chemicals, etc. by thermoforming sheets thereof to form containers.
However, polypropylene has a poor transparency, and there have been proposed many methods of improving the transparency of polypropylene by, for example, quenching an extruded resin sheet, by adding nucleating agents, or by adding a petroleum resin. Among them, the method of adding a petroleum resin can provide sheets excelling in transparency and moldability, but it has still left many problems such as insufficient low temperature impact strength of the produced sheets and drawdown caused by the indirect heating during thermoforming.
For example, in the method disclosed in Japanese Patent Application Publication No. 53-15894, polypropylene-based sheets are prepared by melting a composition comprising a polypropylene and an aliphatic or alicyclic petroleum resin at a temperature higher than the melting point of the polypropylene by 40.degree. C. or more, followed by molding the molten composition by quenching it to a temperature of not higher than 60.degree. C. by means of a chill roll method, an under-water method, etc. However, the obtained polypropylene-based sheets still have an insufficient transparency and a poor low temperature impact resistance, and furthermore a drawdown occurs during thermoforming the sheets.
The other methods disclosed in Japanese Patent Application Laid-open Nos. 53-117069 and 55-27203 attempt the improvement of the transparency of polypropylene-based sheets by adding a petroleum resin to a polypropylene-based resin and rolling the obtained sheets. However, the methods cannot attain sufficient improvement of transparency without a rolling of a high extent which causes decreases of the moldability and the low-temperature impact resistance of the sheets due to the orientation caused therein.
There has been a method for improving the low temperature impact resistance of a polypropylene-based resin by adding a low-crystalline or amorphous resin, such as an EPR, an ethylene-butene-1 copolymer, an SBR, and the like, to a polypropylene-based resin. The addition of such resins, however, results in a decrease in transparency or rigidity of the obtained resin sheets. In addition, there is also known a method for improving the thermoforming moldability of a polypropylene-based resin by adding an LDPE or an HDPE to a polypropylene-based resin. But this method also results in a decrease in transparency or rigidity of the obtained resin sheets.